242 N. L. Bowen — The Ternary System : 



free silica. Since the microscope showed inhomogeneity, and 

 since the forsterite was not recognized as such, but was con- 

 sidered to be the high-temperature form of MgSi0 3 (a-MgSi0 3 ), 

 these mixtures were considered to lie beyond the limits of solid 

 solution. * 



The thermal results obtained seemed to be in accord with 

 these conclusions, for these more magnesian preparations gave 

 two heating-curve breaks, one of which appeared to be inter- 

 pretable as the result of inversion of /3-MgSi0 3 to a-MgSi0 3 

 and the other, as the result of eutectic-melting between MgSi0 3 

 and diopside solid solution. 



The heating-curve breaks obtained on the more calcic mix- 

 tures (those that crystallized to homogeneous pyroxenes) 

 should indicate the temperature of equilibrium changes and 

 should therefore agree with the quenching results. This they 

 do. Heating-curve results obtained on the more magnesian, 

 inhomogeneous material, not itself at equilibrium, may indicate 

 other changes. The completed results of the ternary system 

 make it possible, however, to state what effects should be 

 observed in this inhomogeneous material. 



Since the material contains free silica and medium-calcic 

 pyroxene there should be a notable melting-together of these 

 two at temperatures corresponding to the flat portion of the 

 boundary curve tridymite-pyroxenes (1370°-1380°; see fig. 7). 

 Since forsterite is present also there should be melting of it 

 with a further quantity of pyroxene at temperatures corre- 

 sponding to the flat portion of the boundary curve pyroxene- 

 forsterite (1387°-1389°). The melting should in each case 

 give a heating-curve break and it was approximately at these 

 temperatures that breaks w T ere obtained in the earlier work. 

 As already pointed out, the lower of these two breaks was at 

 that time considered probably due to inversion in MgSi0 3 , but 

 this involved the difficulty that the break could not be obtained 

 in MgSi0 3 itself. It has been shown by Bowen and Andersen 

 that this suspected inversion in MgSi0 3 has no real existence 

 inasmuch as the material called a-MgSi0 3 has proven to be 

 forsterite Mg 2 Si0 4 . The break cannot, therefore, be the result 

 of inversion. The presence of the break in all mixtures con- 

 taining lime and its absence in pure MgSi0 3 is, however, in 

 direct accord with the conclusion, stated above, that the break 

 is due to the melting-together of silica and medium-calcic 

 pyroxene. If either silica or calcic-pyroxene is absent the 



* The amount of silica is very small (2-3 per cent). It occurs in exceed- 

 ingly minute particles speckled through some of the other crystals, the 

 effect "being merely to impart a dusty appearance to these crystals. It is to 

 be noted that these minute inclusions were observed to be ''more numerous 

 in the mixtures which are rich in magnesia (70-9? per cent).'' 



